This invention relates to the fabrication of microminiature structures and, more particularly, to an etching apparatus and to a processing method for making very-large-scale-integrated (VLSI) devices.
Considerable interest exists in employing dry processing techniques for forming patterns in workpieces such as semiconductor wafers. The interest in dry processing stems from its generally better resolution and improved dimensional and shape control capabilities relative to standard wet etching. Thus, dry etching is being utilized increasingly for, for example, fine-line pattern delineation in the processing of semiconductor wafers to form VLSI devices.
Various dry etching processes that involve radio frequency (rf)-generated plasmas in a reaction chamber are known. These so-called plasma-assisted processes include reactive sputter (or ion) etching. In reactive sputter etching, the workpieces to be patterned are placed on the rf-driven cathode electrode in the reaction chamber. In another plasma-assisted process, typically referred to simply as plasma etching, the workpieces are placed on the grounded anode electrode in the reaction chamber. These and other processes suitable for patterning layers in VLSI devices are described by, for example, C. M. Melliar-Smith and C. J. Mogab in "Plasma-Assisted Etching Techniques for Pattern Delineation," Thin Film Processes, edited by J. L. Vossen and W. Kern, Academic Press, New York, 1978, pp. 497-552.
In some standard dry etching processes, pattern delineation is achieved by covering selected portions of a layer with a relatively etch-resistant masking pattern. The uncovered portions of the underlying layer to be patterned are then subjected to the etching plasma and thereby removed.
In other dry etching processes, pattern delineation in a negative-tone resist layer is achieved without the use of an overlying masking pattern. Such processes are described, for example, in U.S. Pat. No. 4,232,110, assigned to the same assignee as is the present application, and in a copending commonly assigned U.S. application designated Ser. No. 256,604, filed Apr. 22, 1981 for G. N. Taylor.
In these last-mentioned processes, a film of resist material comprising a host polymer and one or more monomers is selectively irradiated to reduce the mobility of the monomer or monomers in the irradiated regions. This mobility reduction step is referred to as "locking." The film is then fixed, typically by heating, in a vacuum or not, to substantially remove the unlocked monomer from the unirradiated regions. Subsequently, the film is dry etched in a plasma, typically in an oxygen plasma in a reactive sputter etching step. The locked monomer reduces the rate of etching in the irradiated regions relative to that in the unirradiated regions. Accordingly, when the unirradiated regions are etched down to an underlying substrate, a negative resist pattern comprising the irradiated regions remains on the substrate.
Heretofore, plasma-assisted etching processes designed to pattern micron and sub-micron features have often been plagued in practice with a number of problems. Thus, nonuniformity of etching across the surface of the wafer to be patterned often occurs. This results, for example, from the application of nonuniform electric fields to the surface. In particular, nonuniform fields at the edges of the wafer typically cause the etching rate across the surface of the wafer to be nonuniform.
Another obstacle to achieving better results in plasma-assisted dry etching processes has been the seemingly unavoidable presence of contaminants in the reaction chamber of the etching apparatus. These contaminants constitute, for example, material etched away from various surfaces in the reaction chamber in the vicinity of the wafer(s) to be etched. Or they constitute chemical fragments generated in the chamber as a result of field-induced reactions at surfaces in the vicinity of the wafers. Such contaminants can, for example, deposit on the surface of a layer to be etched and thereby effectively inhibit etching of the portions of the layer that underlie the deposited contaminants. As a result, the pattern etched in the contaminated layer may not be a precise reproduction of a prescribed pattern. Or some of these unetched portions, constituting slivers or so-called "grass" regions, may break off or be transported laterally or penetrate subsequent layers during the device fabrication sequence, thereby causing failures in the devices.
Moreover, field-induced reactions at surfaces in the vicinity of the wafers to be etched may so deplete the active etching species in the chamber that the rate of etching the wafer surfaces is deleteriously affected.
For these and other reasons, considerable efforts have been directed by workers in the art aimed at trying to achieve more uniform, more contamination-free and more rapid plasma-assisted dry etching of wafers. It was recognized that such efforts, if successful, would decrease the cost of devices made in accordance with a fabrication sequence that included such an improved etching process.